We have examined the significance of the recent crystal structure of bovine opsin complexed with an 11-mer peptide sequence derived from the carboxyl terminus of the retinal G-protein transducin (Gt) alpha subunit chain. We synthesized the L341 mutant sequence of the published crystal structure as well as the wild-type sequence for the C-terminal 11 residues of Gt-alpha to test for crystallization of the rhodopsin from the cephalopod mollusc Sepia officinales. Neither peptide stabilized Sepia rhodopsin in dodecylmaltoside or octylglucoside solution. In contrast, the C-terminal sequence of Gq protein, the cognate G-protein for cephalopod visual response, increased the rate of thermal denaturation of Sepia rhodopsin. Thus, these sequences were not pursued for obtaining crystals with Sepia rhodopsin. Additionally, we tested the functional interactions of the C-terminal peptides with rhodopsins. The L341 peptide displayed a Ki of 3 10-5 M for inhibiting bovine rhodopsin activation of Gt while the wild-type sequence inhibited with a Ki of 6 10-4 M. These values were unchanged with variation of the concentration of Gt-alpha, indicating a non-competitive mechanism for the inhibition. Further, neither peptide displayed the expected competitive interaction when Gt-alpha was varied at several fixed concentrations of the peptides. Saturation analyses of the inhibition as a function of temperature revealed that the L341 mutant and K341 wild-type sequences inhibit by different chemical interactions, the L341 peptide saturating with a shallow profile (Hill slope 0.6) and being strongly hydrophobic while the K341 wild-type sequence saturates with a steep slope (Hill slope 2.4) and dominantly enthalpic. Thus, neither peptide interacts at the Gt-alpha binding site of rhodopsin;and the L341 mutant does not reflect the same interaction as the native K341 sequence. Therefore, we will not pursue these sequences further for structural studies. Previously, we had constructed an ensemble of chimeric G-alpha subunits using G-alpha-i1 as the backbone and introducing sequence from G-alpha-q or G-alpha-s to test for the sequences within G-alpha conferring receptor-selectivity. Because of the crystal of opsin with the C-terminal 11-mer mutant sequence from G-alpha-t, we examined the G-alpha-i1/q chimera containing the terminal 11 amino acid residues contributed by G-alpha-i1/q11. Tested with the human 5HT2c receptor, the results we have obtained are ambiguous as to the contribution of this sequence for receptor recognition. While this chimeric G-alpha-i1/q11 is not recognized as being quantitatively identical to G-alpha-q, we can not rule out some enhancement of recognition compared with G-alpha-i1. Our future work will test this and a similar G-alpha-i1/s11 construct using receptors that show no measurable activation of G-alpha-i1, as opposed to the human 5HT2c receptor which recognizes G-alpha-i1 poorly, but measurably.